Once a growing tumour has been removed the degree of immunity of the host, as determined by in vitro tests and the capacity to reject autografts, is very much greater than in the presence of the growing tumour. Attempts will be made to define the cause of depressed immunological responsiveness in tumour-bearing animals by analysis of the different components of cellular immunity (i.e. the specific cytotoxic activity of immunoblasts, sensitized small lymphocytes and "armed" macrophages) in the tumour-draining lymph nodes, distant lymphoid tissue, peripheral blood, the thoracic duct lymph and peritoneal exudate before and after tumour removal. In addition there will be an investigation of factors in the serum of these animals, in particular circulating antigen, which may interfere in the expression of cytotoxicity of immune lymphoctes and macrophages.